Storage tanks are commonly used to store various types of petroleum products. The storage tanks can range in volume capacity from a few thousand cubic meters to hundreds of thousands (or more) of cubic meters. Since the storage capacity of such tanks is known before hand, the tanks are frequently used to measure the amount of petroleum product held in the tanks. However, the volume capacity of such tanks does not remain fixed, but changes with the amount of product placed in the tanks, as well as other factors that affect volume capacity, such as, for example, tank geometry, material used for the walls of the tanks, ambient temperature, and ambient pressure, among other things. Therefore, it is common practice to calibrate storage tanks in order to determine accurate volumetric capacity and, resultantly, accurately determine the amount of petroleum product (e.g., oil or gas) in the tanks.
There exist a number of methods of calibrating or measuring the volume of large storage tanks. For instance, one known method is to fill a tank and then measure the amount of liquid drained from the tank. This method, however, is very time consuming, and can be very costly for large size tanks. Normally, this method is avoided unless the tank volume cannot be determined geometrically through physical measurement of the tank parameters.
Another method for calibrating tanks is called the optical reference line method (ORLM). The ORLM provides for the calibration of cylindrical tanks by measurement of one reference circumference, followed by determining the remaining circumferences at different elevation levels on the tank. The remaining circumferences are determined by measuring the horizontal offset of the tank wall from a vertical optical reference line. These circumferences are corrected, based on wall thickness, to calculate true internal circumferences, which can then be added to determine the tank volume.
An example of an ORLM method is shown in FIG. 1, in which there is shown a tank 2, a magnetic trolley 4, an optical device 6, and a horizontal graduated scale 8 attached to the trolley 4. During operation, the optical device 6 produces an optical ray beam 10 upwardly and parallel to the tank wall 12. The magnetic trolley 4 is typically controlled by an operator 11 positioned on top of the tank 2, who holds a rope 13 attached to the trolley 4. The operator 11 pulls or releases the rope 13 to move the trolley 4 up or down along the tank wall 12.
In order to determine volume, a reference circumference C is initially measured along the perimeter of the tank 2. The reference circumference C is measured using a measuring tape (not shown), and is typically measured near the bottom of the tank 2. With the reference circumference C known, the trolley 4 can be raised or lowered by the rope 13 to various vertical stations V along the tank wall 12. In most systems, the vertical stations V are located between the weld seams on the tank 2. In FIG. 1, two of the vertical stations are indicated by lines V. At each vertical station V, the horizontal offset between the tank wall 12 and the optical ray beam is noted using the horizontal graduated scale 8.
Once a series of measurements have been taken at the vertical stations V, the measurements are repeated with the optical device 6 rotated 180-degrees to verify accuracy. Thereafter, the measurements are used to determine the circumference of the tank at each vertical station V (using the reference circumference as a reference point), and the volume of the tank 2. Additional factors can also be considered when calculating volume, such as, for example, the temperature of the tank walls 12. This temperature is typically derived based on the temperature inside the tank and the ambient temperature.
While the ORLM method shown in FIG. 1 is better in some ways than filling the tank 2 and measuring the fluid drained from the tank 2 to determine volume, as discussed above, it has significant drawbacks. For example, measuring the horizontal offset of the trolley 4 from the optical ray beam 10 at only a few select vertical stations V provides relatively few data points from which tank circumferences can be measured. Although this data can be extrapolated to estimate the volume of the tank 2, such extrapolations tend to be inaccurate. Additionally, the ORLM method shown in FIG. 1 requires the operator 11 to be positioned on the top of the tank, which can be dangerous. Furthermore, the use of the optical ray beam 10 and a horizontal graduated scale 8 to measure the horizontal offset of the tank wall 12 lacks the precision necessary to calculate accurate tank volumes. This is because an operator must read the horizontal graduated scale 8 at each horizontal offset, often from a distance.
To overcome drawbacks related to the operator having to read the horizontal graduated scale 8 at each horizontal offset, it is known to replace the horizontal graduated scale 8 on the trolley 4 with a linear position sensor that accurately senses the location where the optical ray beam 10 impinges on the linear position sensor, and, thereby, facilitates accurate determination of the circumference of the tank wall 12 at the measurement location (e.g., vertical station V). Such linear position sensors, however, fail to sense the optical ray beams 10 where significant drift occurs between the optical device 6 and the trolley 4, such as, for example, due to irregularities or deformations in the tank wall 12. When this happens, horizontal offset measurements cannot be made at such measurement locations, and the inaccuracies introduced into the volumetric capacity calculations by the missing measurements can be great enough to render the ORLM calibration method unreliable.
There exists an unfulfilled need for an apparatus, a system and a method that provides self-calibration for offset measurements and that overcomes the disadvantages of known systems.